A fuel cell system is an electric power generation system, operative to achieve electrochemical reaction between hydrogen, obtained by reforming fuel such as natural gas, and oxygen in air for directly generating electric power, which is able to effectively utilize chemical energy owned by fuel and has characteristics friendly to environments, and research and development work has been undertaken in a full scale to commercially apply the fuel cell system into practical use.
When stopping the operation of the fuel cell system, first, external loads, such as a drive motor, are disconnected from a fuel cell body. When this takes place, since no load current flows, no load voltage appears in a voltage across an anode electrode and a cathode electrode of the fuel cell body, resulting in a high voltage condition that exceeds a value of 0.8V per unit cell. If a platinum (Pt) catalyst of a fuel cell is exposed to such a high voltage condition, the ionization of platinum rapidly occurs, causing dissolving degradation of platinum to occur in the cathode electrode. The ionized platinum readily couples to oxygen to form oxidized platinum (PtO), which provides no contribution to electrical power generation with remarkable reduction in an effective active area of the catalyst.
To prevent such degradation, the voltage of the fuel cell body needs to be lowered as fast as possible. To this end, with a fuel cell system disclosed in Japanese Patent Provisional Publication No. 6-333586, connecting a dummy resistor to a fuel cell during a halt thereof causes a load current to be extracted from a fuel cell body to induce a drop in a voltage. When this takes place, hydrogen is continuously supplied to an anode electrode so as to avoid a shortage of hydrogen while stopping the supply of oxygen to a cathode electrode to allow residual oxygen to be consumed for drop in voltage.